1. Field of the Invention
The present invention relates to an image generating apparatus such as printers, copying machines and fax machines, and more particularly, to an improvement of an image generating apparatus with a thermal fusing apparatus using a thermal roller system or film heat system as an apparatus for fusing an unfused image formed and borne on a recording medium.
2. Description of the Related Art
As a fusing apparatus (sometimes called “fuser”) for the image generating apparatus such as a copying machine and a printer, a thermal roller system or film heat system has been known conventionally.
In particular, the thermal fusing apparatus based on the film heat system has an advantage of being able to suppress the power consumption to an extremely low level because it does not require power supply during standby, thereby offering an effective energy-saving, on-demand fusing apparatus. The thermal fusing apparatus based on the film heat system is disclosed in Japanese Patent Application Laid-open Nos. 63-313182 (1988), 2-157878 (1990), 4-044075 (1992), and 4-204980 (1992).
The thermal fusing apparatus based on the film heat system basically includes a heater (heating component, heating body) fixed to a supporting body; a fusing component including a heat resistant thin film (fusing film) sliding on the heater; and a press roller as a pressing component for forming a fusing nip by press-contacting the heater via the film. The fusing nip pinches and transports the recording medium on which the unfused image is formed so that the unfused image is thermally fused as a permanent image by the heat fed from the heater via the film.
The fusing apparatus based on the film heat system can reduce the heat capacity of the heating body itself and of the film placed between the heating body and recording medium as compared with the other well-known fusing apparatuses such as a thermal roller system. Accordingly, it is superior to them in terms of power saving and quick start (reduction in wait time).
As the heater, a ceramic heater is generally used. For example, ceramic substrates of alumina (Al2O3), aluminium nitride (AlN) and the like are used as the heater substrate, because they have such properties as high electric insulation, good heat conduction and low heat capacity. On the heater substrate, a heating resistance layer composed of silver palladium (Ag/Pd), Ta2N or the like is formed by screen printing, followed by coating the surface of the heating resistance layer with a thin glass protective layer. As for the ceramic heater, its heating resistance layer is heated by the current flowing through it so that the heater increases its temperature sharply in its entirety including the ceramic substrate and glass protective layer. The temperature rise of the heater is detected by a thermistor, a temperature detecting means, which is disposed at the back of the heater, and is fed back to a current controller. The current controller controls the current to be supplied to the heating resistance layer such that the heater temperature detected by the thermistor is maintained at approximately a specified temperature (fusing temperature). Thus, the heater is heated and controlled at the specified fusing temperature.
The thermistor disposed at the back of the heater is usually an NTC thermistor or the like. An example of a heat resistant, high insulating structure is shown in FIG. 1. It has a thermistor unit that includes a heat resistant, heat-insulating, elastic ceramic fiber layer 76b, the so-called “ceramic paper layer”, a polyimide thin film 76d, and an NTC thermistor 76c disposed between them. The thermistor unit is pressed to the back of the heater with appropriate pressure. Alternatively, a structure (not shown) is known which has a conductive pattern formed on the back of the heater, and a thermistor conductively adhered thereto via a heat conduction adhesive.
As for the image generating apparatus including the fusing apparatus, configurations are known which have a plurality of thermistors mounted on the heater in the fusing apparatus to control power of the heater using them as disclosed in Japanese Patent Application Laid-open Nos. 5-080604 (1993), 5-080605 (1993), and 5-080665 (1993).
In such an image generating apparatus, when a paper conveyance reference occupies the center of the fusing apparatus, for example, a first thermistor (main thermistor) is placed at the center in the longitudinal direction of the back of the heater (direction perpendicular to the conveyance direction), and second thermistor (sub-thermistor) is placed at a longitudinal end of the heating body. The power control is carried out for maintaining the heater temperature at the target temperature in response to the temperature detected by the main thermistor.